In optical microscopy, most red-emitting dyes provide only moderate performance due to unspecific binding, poor
labeling efficiency, and insufficient brightness. Here we report on four novel red fluororescent dyes, including the
first phosphorylated dye, created by combining a rigidized rhodamine backbone with various polar groups. They
exhibit large fluorescence quantum yields and improved NHS ester stability. While these fluorophores are highly
suitable for fluorescence microscopy in general, they excel in stimulated emission depletion (STED) microscopy,
providing < 25 nm spatial resolution in raw images of cells.